This invention relates to the elimination of a caustic prewash in the fixed bed sweetening of hydrocarbon feedstocks containing a high level of naphthenic acids and to the recovery of the naphthenic acids. More particularly, it relates to the use of aqueous ammonia as an adjunct to fixed bed sweetening in a process where not only the caustic prewash can be eliminated, but the naphthenic acids can be recovered and the ammonia values also can be recovered and reused. Because of the rather particularized nature of our invention, it appears desirable to expound on certain current process characteristics so that the contributions of the present invention in advancing the relevant art can be better appreciated.
Many hydrocarbon streams have sulfur-containing compounds as undesirable components whose removal constitutes an important stage of hydrocarbon processing. Where these components are mercaptans their "removal" is generally only a conversion of mercaptans to disulfides which remain in the feedstock as inoffensive components of the hydrocarbon stream, a process usually referred to as "sweetening" (with the initial mercaptan-laden stream referred to as "sour" feedstock). The conversion of mercaptans to disulfides often is accomplished merely through air oxidation as catalyzed by various metal chelates; see J. R. Salazar in "Handbook of Petroleum Refining Processes", R. A. Meyers, editor, pages 9-3 to 9-13. But catalysis of mercaptan oxidation proceeds best in an alkaline environment--and therein hangs our tale.
The prior art has required a highly alkaline environment, typically achieved by strong bases such as alkali metal hydroxides (for example, caustic soda). Unfortunately, the caustic does not merely provide an alkaline environment but in time is neutralized by acidic components of the hydrocarbon stream, requiring its continued replacement and replenishment. Disposal of spent caustic solutions is itself an environment problem, and proper disposal may exact a heavy financial penalty on the sweetening process. This is especially true for certain feedstocks, such as kerosine, which typically have a significant content of naphthenic acids.
Naphthenic acids are carboxylic acids found in petroleum and various petroleum fractions during their refining; see Kirk Othmer, "Encyclopedia of Science and Technology", 3rd Edition (1981), pp 749-53. Naphthenic acids are predominantly monocarboxylic acids having one or more cycloaliphatic groups alkylated in various positions with short chain aliphatic groups and containing a polyalkylene chain terminating in the carboxylic acid function. Although cyclopentane rings are the predominant cycloaliphatic ring structure, other cycloaliphatics rings, such as cyclohexanes, also may be present in appreciable quantities. The predominant acids are represented in Kirk Othmer by the formula, ##STR1## where n may range from 1 to 5, m is greater than 1, and R is a small aliphatic group, predominantly a methyl group. Since naphthenic acids are well known in the art their further characterization is unnecessary and the interested reader may consult appropriate texts for additional information.
The naphthenic acid content of feedstocks such as kerosine engenders further complications arising from the limited solubility of alkali metal naphthenates in concentrated alkali. One consequence is that when a caustic-wet fixed bed oxidation catalyst is used--a common and otherwise economically favored variant--formation of insoluble alkali metal naphthenates tends to cause bed plugging. To avoid this, kerosine and kerosine-like feedstocks undergo a caustic prewash to remove naphthenic acids prior to entry of the feedstock to the fixed bed. But the solubility characteristics of the alkali metal naphthenates are such that their efficient extraction from kerosine-type feedstocks into aqueous media requires utilization of a dilute caustic (usually under 3 weight percent) prewash, which increases the volume of the spent caustic and further intensifies its disposal problem.
Although naphthenic acids are troublesome in the sweetening process they do have significant value as precursors to wood preservatives, oil-based paint dryers, surfactants, corrosion inhibitors, and lubricant additives. Their recovery is highly desirable, but in the scenario described above they must be recovered from a dilute aqueous solution, which imposes yet another financial burden.
The dilemma faced by a processor with the need to sweeten the liquid hydrocarbon feedstocks, and especially kerosine-type feedstocks, is multifaceted. The most desirable sweetening process which converts mercaptans to disulfides operates best in an alkaline environment. The naphthenic acids in feedstocks previously have been removed in a caustic prewash to avoid reactor bed plugging, but the limited solubility of alkali metal naphthenates requires the use of dilute alkali, which exacerbates the disposal problem of spent caustic solutions. Although the naphthenic acids themselves are valuable commodities whose recovery might otherwise offset spent caustic disposal costs their recovery from dilute alkali is difficult and expensive, with little if any economic return. The result is that high naphthenic acids in a hydrocarbon feed complicate a simple chemical process with economic burdens.
The villains in this drama are not the naphthenic acids; basically these are quite desirable articles of commerce. Instead the villain is caustic. Heretofore this villain was perceived as omnipresent and unavoidable, truly a necessary evil. But our invention is but another example of the triumph of good over evil, for we have found a way which at once avoids the villain of caustic solutions while capturing the naphthenic acids in a gilded monetary net.
The keystone of our invention is the observation that if high naphthenic acid unsweetened hydrocarbon feedstocks are mixed with aqueous ammonia prior to entering a fixed bed oxidation catalyst effecting sweetening, there is no formation of insoluble naphthenate salts causing bed plugging. Evidently the solubility of ammonium naphthenates relative to alkali metal naphthenates is enough greater to obviate the problem of bed plugging. This property in itself permits one to eliminate a caustic prewash. In addition the aqueous phase can be separated from the hydrocarbon phase after the reaction zone and the ammonia either reused, in whole or in part, and naphthenic acids can be more readily recovered from the aqueous ammoniacal solution than from the dilute caustic resulting from a caustic prewash.
Although recovery of a component from a concentrated solution would be expected to be significantly easier than its recovery from dilute solutions, the recovery of naphthenic acids from ammoniacal solutions of their ammonium salts is expedited still further by the fact that heating the ammoniacal solution causes the decomposition of the soluble (or, perhaps more accurately, the colloidal dispersion of) ammonium naphthenates to insoluble naphthenic acids. Thus, heating the recovered aqueous phase precipitates naphthenic acids which can be readily recovered in a quite high yield simply by, for example, filtration or centrifugation. As a bonus ammonia also is separately recoverable for reuse. In some cases distillation of all, or most, of the water may be desirable for optimum recovery of the naphthenic acids and ammonia. The result is not only the elimination of the disposal problem attending a caustic prewash but virtually quantitative recovery of valuable naphthenic acids at little expense and at little additional cost, with the added bonus of ammonia reuse. In addition, the lower base strength of ammonia relative to caustic may lead to more selective removal of naphthenic acids relative to phenols, a rather desirable result. The overall economic benefits can not be underestimated.